New 4.0 OHV Heads - Now hesitates around 40 MPH - Please Help!

[Jim Jermain]

I recently replaced the heads on my 2000 4.0 OHV. It fixed my cold start rough idle problem, but now I am getting a slight stumble under light acceleration typically going up a hill. It doesn't tend to stumble if I accelerate harder. I've changed and tried the following (some were prior to replacing the heads). I don't get and codes.

Plugs and wires (about 20K miles ago)
Upper intake, lower intake
EGR valve
EGR Pressure sensor
Cleaned Fuel injectors
New fuel filter
Cleaned MAP
new TPS
New IAC
Checked fuel pressure about 5K miles ago
Smoke tested intake (detected some smoke coming out of egr valve - replaced valve)

I am totally stumped and frankly getting really tried of this Explorer. Please help!! I just don't know what to do next.

Thanks,

Jim

 

[jd944]

Try disconnecting and plugging the EGR vacuum line to see if it goes away.

 

[Jim Jermain]

Thanks for the tip. I will give it a try. After I posted the question, I remembered I replaced the EGR solenoid, not the differential pressure sensor. I've never received any codes on the EGR, but maybe the differential pressure sensor is going bad.

The thing that really has me baffled is this stumble only happened after I changed the heads. I can't think of how changing the heads would cause this to occur unless when I parents owed the car a dealer they took it to remapped the computer in an attempt to fix the rough idle that was caused by the cracked heads and now that the heads are replaced it needs to be remapped.

I did change the axle ratio (went from a 3.55 to a 3.73) about a year ago. I don't know again if changing the heads now would somehow cause a stumble because I changed the rear axle ratio a year; that seems like a stretch.

Last night a drove the car for about 170 freeway miles. I don't notice the stumble - hesitation when I start from a stop. It will accelerate very smooth and strong. It's only when I am accelerating at high speeds (typically over 40 mph) or trying to maintain speed as I climb a hill. This is when the engine will stumble - hesitate - run rough.

I am really stumped. I really appreciate any tips - suggestions.

Thanks guy!!!

Jim

 

[swetrid]

I would double ck the plug wires, once you remove sometimes they break or crimps are loose and cause miss. Also double ck your routing on driver side with egr tube area.

 

[Jim Jermain]

Thanks I to am wondering about the plug wires. Although they are relatively new I did find I had a slight rough idle after replacing the heads all the time versus just at cold start before I replaced the heads. I pulled out all the spark plugs and checked the gap. When I reinstalled the engine ran rough and I had a misfire on cylinder one. I replaced the plug and checked for spark. Once I did this the rough idle and misfire went away. I thought I had solved all my problems. The idle is very smooth, but I still get the stumble.

I did try disconnecting the EGR valve and plugging the vacuum line. No change, I still get the stumble. I may try replacing the plug wires and plugs one more time. If that doesn't do it, I think it is off to the dealer for an analysis.

Any thoughts on a recommended plug for this engine?

Thanks,

Jim